1. Field of the Invention
This invention relates to a cylindrical magnetron sputtering system and, more particularly, to a cylindrical hollow cathode magnetron sputtering system for depositing a film or films on cylindrical substrates.
2. Description of Related Art
Thin layers of conducting materials such as metals, metal silicides, or low-resistivity polycrystalline silicon; and insulating substances such as silicon dioxide, silicon nitride, and phosphosilicate glass constitute important elements of many semiconductor devices.
Various techniques have been developed to cause such layers to be established as part of a device. One such technique, known as film deposition, involves supplying component materials for a growing layer from external sources and depositing those materials down upon a substrate. Such deposition processes are generally carried out in a vapor phase within a reduced pressure atmosphere of a selected gas or gases, or in a vacuum. If the material to be deposited does not react chemically during deposition, the process is referred to as Physical Vapor Deposition or PVD. If, on the other hand, the deposited material is a product of a chemical reaction which occurs within the vapor phase, either on the surface or in the vicinity of the substrate, the process is known as Chemical Vapor Deposition or CVD. Hybrid methods of film deposition, i.e., those which involve both physical and chemical processes, are also known.
One method of physically depositing a film upon a substrate is known as sputtering. A typical sputtering system includes a target (a cathode) and a substrate holder (an anode) positioned so that the surface of a substrate upon which the film to be deposited, which substrate is placed on the holder, faces the target. The target is a plate of the material to be deposited or from which a film is to be synthesized. The target is connected to a negative voltage supply, either dc or rf, and the substrate holder may be either grounded, floating, or biased, as well as either heated, cooled, or some combination thereof. A gas, at a pressure from a few millitorr to about 100 mTorr, is introduced into a chamber containing the substrate holder and target to provide a medium in which a glow discharge plasma can be initiated and maintained. When the glow discharge is started, positive ions strike the target and stimulate the removal of mainly neutral target atoms therefrom by momentum transfer. These atoms then condense into a thin film formed upon the surface of the substrate placed on the substrate holder. In addition, various particles other than neutral atoms, e.g., electrons and ions, are also produced at the target which may have a significant effect on the properties of the film deposited on the substrate.
Examining the sputtering process in more detail, a low pressure abnormal negative glow plasma discharge is maintained within the chamber between the cathode (target) and the anode (substrate holder). Electrons emitted from the cathode due to ion bombardment thereof are accelerated to near the full applied potential within the cathode dark space, i.e., a relatively nonluminous region between the cathode and the negative glow. Such high energy electrons enter the negative glow as so-called primary electrons where they collide with gas atoms and produce the ions required to sustain the plasma discharge. The primary electron mean free path increases with both increasing electron energy and decreasing pressure within the chamber. At low pressures, ions are produced far from the cathode where their chances of being lost are great. Additionally, many primary electrons hit the anode with high energies, causing a loss that is not offset by impact-induced secondary emission. Thus, ionization efficiencies are low. As the pressure within the sputtering chamber is increased at a fixed voltage, the primary electron mean free path decreases and larger currents are possible; however, at high pressures within the chamber the sputtered atom transport which occurs has been found to be reduced by collisional scattering.
It has also been found that a magnetic field extending parallel to the cathode surface can restrain primary electron motion to regions in the vicinity of the cathode and thereby increase ionization efficiency. It has been further found that the E.times.B electron drift currents can be caused to close on themselves by the use of cylindrical cathodes, which thereby prevent the E.times.B motion from causing the plasma discharge to be swept to one side. Based upon the foregoing, various cylindrical magnetron systems have been developed. Such systems having cylindrical, hollow cathodes are known as inverted magnetrons or cylindrical hollow magnetrons. A typical cylindrical hollow magnetron sputtering system includes one or more solenoids, wound on a core of magnetic material, and placed coaxially and externally to or within the cathode to serve as a field generator. Typically, the anodes are also joined to tubular backstrap and are both made from magnetic material. The aforementioned anode design effectively reduces field curvature near the ends of the anode and also increases the magnetic field strength in the plasma located inside the cathode. Where a plurality of solenoids are used, current ratios of those solenoids may be controlled to provide a variety of field shapes. To avoid changes caused by unequal heating of the solenoids, they are also typically connected in series with one another.
Heretofore, cylindrical hollow magnetron systems have been recognized as useful for coating substrates of complex shapes where: (a) the hollow cathode has a uniform wall erosion rate; (b) the substrate surface is far enough from the ends so that end losses can be ignored; and (c) the object to be coated has an unobstructed view of the cathode surface. Thus, heretofore, the usefulness of cylindrical hollow magnetron sputtering systems has been viewed as involving positioning the anode where end losses may be ignored and positioning the object to be coated so that it is always completely exposed to the cathode surface.
The deposition of thin film coatings onto cylindrical substrates such as wires and fibers, which have not been recognized as having complex shapes, has heretofore involved either rotation of the substrate while moving it relative to a uni-directional coating material source, such as a planar diode, or other steps wholly unrelated to the processes described herein. Needless to say, those prior processes which involve rotating a wire or fiber being coated with a thin film require the use of complex rotating means. Even using the most precise rotating systems now available cannot ensure a film of sufficient uniformity of thickness and quality for a number of currently developing applications. For example, it is becoming highly desirable to be able to deposit films of a few microns in thickness upon optical fibers, ceramic fibers, thin wires and other such cylindrical substrates. Certain new applications require the deposit of one or more films of metallic, superconducting, dielectric, electro-optic, magnetic and/or piezoelectric materials onto the surface of fibers and wires in highly precise and uniform layers.
In addition to the problems of film thickness and uniformity discussed above, the prior art methods of film deposition include a number of other shortcomings which render them inefficient in coating cylindrical substrates. For example, it has been found that using a planar magnetron sputtering system to apply relatively thick films over very large lengths of fibers is extremely inefficient because of the inherently low cathode material utilization characteristic of such systems when thin fibers are used as substrates.
The method and system of the present invention overcomes many of the disadvantages of prior art sputtering systems when the substrates to which a film is to be applied are wires and fibers.